The present invention relates to a method of manufacturing a shadow mask for a color picture tube and, more particularly, to a shadow mask manufacturing method using photoetching.
The present invention also relates to a cleaning device used in a shadow mask manufacturing process.
Furthermore, the present invention relates to an apparatus for manufacturing a shadow mask.
As shown in FIG. 1, a shadow mask type color picture tube has a vacuum envelope 23 consisting of a panel 1, a cone 20, and a neck 21. In this vacuum envelope 23, a phosphor screen 2, a shadow mask 3, and an electron gun 4 are arranged. The phosphor screen 2 is formed on the inner surface of the panel 1 and consists of three kinds of phosphor layers emitting three different colors, respectively. The shadow mask 3 is arranged as a color selection electrode apart from the phosphor screen 2 by a predetermined distance and has a large number of apertures arranged in a predetermined manner and having a predetermined shape. The electron gun 4 is provided in the neck.
In a shadow mask type color picture tube, this shadow mask 3 selects three electron beams 5 emitted from the electron gun 4 so that these electron beams correctly land on the respective predetermined phosphor layers.
The phosphor screen 2 has phosphor dots or stripes and a black matrix burying the portions between these dots or stripes (none of them is shown). This black matrix absorbs landing errors of the electron beams 5 and improves the contrast.
The shapes of the apertures in the shadow mask 3 are roughly classified into a circle and a rectangle. In principle, shadow masks having circular apertures are used in color display tubes for displaying characters and graphics, and shadow masks having rectangular apertures are used in general home color picture tubes.
Recently, a high definition and a high quality are strongly demanded on color display tubes. Accordingly, efforts are being made to decrease the size of apertures in a shadow mask and reduce variations in the aperture size. This is because a shadow mask is used in the formation of a phosphor screen. Generally, in color picture tubes, a phosphor screen for displaying images is formed by photolithography by using a shadow mask as a photomask. For this reason, the size and shape of matrix apertures of a black matrix or of dot-like phosphor layers of three emitting colors constituting this phosphor screen grate depend upon the size and shape of apertures in the shadow mask used. Variations in the size and shape of apertures in the shadow mask appear as unevenness of displayed images and degrade the image quality.
Conventionally, the apertures in shadow masks are formed by photoetching. In particular, apertures are usually formed by a two-stage etching process in display tube shadow masks requiring a high definition and a high quality.
FIGS. 2 to 8 are schematic views for explaining a conventional two-stage etching process.
As a substrate of a color display tube shadow mask, a thin metal plate 7 made from, e.g., an invar material consisting of an Fe--Ni alloy containing such as 36 wt % of Ni or aluminum killed steel is used. This thin metal plate 7 is subjected to degreasing and cleaning to remove, e.g., rolling oil and rust preventing oil.
Photosensitive film formation step
As shown in FIG. 2, both two surfaces of the degreased thin metal plate 7 are coated with a photosensitive material made from, e.g., casein or modified PVA. The coated photosensitive material is dried to form resist films 8 as photosensitive films.
Exposure step
As shown in FIG. 3, a pair of masters 9 and 19 are prepared. The master 9 has a pattern corresponding to small apertures formed in the surface of a shadow mask, that faces an electron gun. The master 19 has a pattern corresponding to large apertures formed in the surface of the shadow mask, that faces a phosphor screen. These masters 9 and 19 are attached to the resist films 8 on the two surfaces of the thin metal plate 7. Thereafter, exposure is performed to print the patterns of the masters 9 and 19 onto the resist films 8. Since a variation in the exposure amount in the exposure area has an influence on the pattern formation dimensions of the resist films 8, the exposure amount is controlled within a predetermined range.
Development step
The resist films 8 on the both surfaces of which the patterns are transferred are developed by using a developer consisting of water or water and alcohol, thereby removing unexposed portions. Consequently, as shown in FIG. 4, resist films 10 and 30 having patterns corresponding to patterns of the pair of masters described above are formed.
First etching step
Thereafter, a protective film 31 is prepared. This protective film 31 consists of an etching-resistant resin film made from polyethyleneterephthalate (PET) or casting polypropylene (CPP) and a pressure-sensitive adhesive applied on the surface of the etching-resistant resin film. As shown in FIG. 5, the protective film 31 is adhered by using the pressure-sensitive adhesive to the surface on which the resist film 30 is formed. The surface of the thin metal plate 7 on which the resist film 10 is formed is etched by using a ferric chloride solution as an etching solution. Consequently, small concave holes 12 serving as small apertures to be formed in the surface of a shadow mask, that faces an electron gun are formed in the surface of the thin metal plate 7 on which the resist film 10 is formed.
Etching-resistant layer formation step
Subsequently, the protective film 31 attached on the surface on which the resist film 30 is formed is removed. The resist film 10 on the surface in which the small concave holes 12 are formed is stripped, and the resultant surface is washed with water. Thereafter, as shown in FIG. 6, the surface of the thin metal plate 7 in which the small concave holes 12 are formed and the interiors of these small concave holes 12 are coated with varnish, and the varnish is dried to form an etching-resistant layer 13.
A protective film 11 is adhered to this etching-resistant layer 13.
Second etching step
Thereafter, the surface of the thin metal plate 7 on which the resist film 30 is formed is etched with an etching solution. Consequently, as shown in FIG. 7, large concave holes 32 serving as large apertures formed in the surface of a shadow mask, that faces a phosphor screen are formed on the surface on which the resist film 30 is formed.
Finishing step
The protective film 11 is removed, and the resist film 30 on the surface in which the large concave holes 32 are formed and the etching-resistant layer 13 on the surface in which the small concave holes 12 are formed are stripped off using an aqueous alkali solution. Consequently, as shown in FIG. 8, the small concave holes 12 and the large concave holes 32 communicate with each other to form apertures 14.
A shadow mask is manufactured through the steps described above.
Although this method is generally used, the method has the problem of variations in the size and shape of apertures in a shadow mask. This is caused by some factors described below.
First, etching reproceeds by the etching solution remaining in the concave holes 12 and 32 during cleaning after etching.
This reproceeding will be described below with reference to FIG. 9 by using the large concave hole 32 as an example. FIG. 9 is a view for explaining the condition of a thin metal plate immediately after the second etching step. After the second etching step, as shown in FIG. 9, an opening diameter De of the concave hole 32 is larger than an opening diameter Dr of the resist film 30 due to side-etching. As a result, an overhanging portion 15 of the resist film 30 is formed around the opening of the concave hole 32. A relatively large amount of etching solution 16 remains inside the overhanging portion 15. The etching solution thus remaining in the concave holes 12 and 32 is difficult to well remove and displace well with wash water within a short time period even by spraying the wash water. Also, the displacement rate of the wash water differs from one concave hole to another.
The influence of the residual etching solution will be described below with reference to FIG. 10. FIG. 10 is a graph showing the relationship between the concentration of the ferric chloride solution and the etching rate. As indicated by a curve 18, as shown in FIG. 10, initially an increase in concentration of the ferric chloride corresponds to an increase in etching rate. The etching rate peaks at a certain level of concentration of the ferric chloride. The etching rate decreases gradually and becomes relatively constant as the concentration increases. A ferric chloride solution with a concentration around the concentration indicated by the broken line is normally used in the etching step for decreasing the variation of the etching rate with respect to the change in concentration of the etching solution. However, if cleaning using wash water is insufficient, the etching solution remaining in the concave holes is diluted with the cleaning solution. The concentration of the diluted etching solution differs from one concave hole to another, and etching reproceeds at an etching rate corresponding to the concentration of the etching solution. When a thin metal plate is exposed to a low-concentration ferric chloride solution diluted by washing after etching for a long time period as described above, the aperture size of the obtained shadow mask changes as shown in FIG. 8. This results in variations in the aperture size and shape and mottling unevenness.
The second factor is poor cleanness of a thin metal plate itself. This cleanness is particularly a problem before the formation of the photosensitive film and after the stripping of the photosensitive film. If the cleanness is poor before the formation of the photosensitive film, satisfactory adhesion may not be obtained between the photosensitive film and the thin metal plate. If the cleanness is poor after the stripping of the photosensitive film, it is likely that coating and filling of the varnish when the etching-resistant layer is formed become nonuniform and no good adhesion is obtained between the etching-resistant layer and the thin metal plate. The cleanness after the stripping of the photosensitive film is especially crucial when the etching-resistant layer is formed in the subsequent step.